Current demands for high density and performance associated with ultra large scale integration of fabricated devices require submicron features, increased transistor and circuit speeds, and improved reliability. As semiconductor processes progress, pattern dimensions such as line width, and other types of critical dimensions, are continuously shrunken. Such demands require formation of device features with high precision and uniformity, which, in turn, necessitates careful monitoring of the fabrication process, including frequent and detailed inspections of the devices while they are still in the form of semiconductor wafers.
The term “specimen” used in this specification should be expansively construed to cover any kind of wafer, masks, and other structures, combinations and/or parts thereof used for manufacturing semiconductor integrated circuits, magnetic heads, flat panel displays, and other semiconductor-fabricated articles.
The term “inspection” used in this specification should be expansively construed to cover any kind of detection and/or classification of defects in a specimen provided by using non-destructive inspection tools or inspection machines. By way of non-limiting example, the inspection process can include generating an inspection recipe and/or runtime scanning (in a single or in multiple scans), reviewing, measuring and/or other operations provided with regard to the specimen or parts thereof using the same or different inspection tools. Note that, unless specifically stated otherwise, the term “inspection” or its derivatives used in this specification are not limited with respect to resolution or size of inspection area.
A variety of non-destructive inspection tools includes, by way of non-limiting example, scanning electron microscopes (SEM), tunneling electron microscope (TEM), atomic force microscopes (AFM), optical inspection tools, etc.
Inspection generally involves generating some output (e.g., images, signals, etc.) for a wafer by directing light or electrons to the wafer and detecting the light or electrons from the wafer. Once the output has been generated, defect detection is typically performed by applying defect detection method and/or algorithm to the output. Most often, the goal of inspection is to provide high sensitivity to defects of interest while suppressing detection of nuisance and noise on the wafer.
A reference die image is often used for detecting defects. For example, in a die-to-die method, the presence or absence of a defect in a location is checked by comparing the pattern at the desired location in an inspected die with the pattern of the same location in another die, e.g., a previously inspected die. A disadvantage of using a reference die image for inspection is that various noises that are caused by different variations, such as, e.g., process variations, mechanical and electrical variations, etc., may be imposed during inspection. In some cases certain noises can interfere with defect detection and decrease the sensitivity of defect detection. Such noises should be considered and disregarded, otherwise detection sensitivity and integrity are hindered. For instance, in a relatively noisy environment, the real defects can be buried within noise and cannot be duly detected. This challenge increases as the design rules shrinks and more and more potential defects are detected.
There is a need in the art for improving the performance of detecting defects of interest.